Also known as: 触手姦, 触手責め
Oral, Vaginal (Penis if male or Futanari), or Anal penetration by Tentacles, to mimic the act of Sex. Colloquially known as "tentacle rape".
Tentacle Sex is considered separate from Sex (with a Penis), Object Insertion, Interspecies, and Bestiality, and should not be used with those tags unless both are present. However, it can and should still be tagged as Vaginal or Anal.
Don't use this tag if the tentacle is just about to be inserted, but hasn't penetrated yet. Use Imminent Penetration instead.